Changes

jobs/gerrit: fix artifacts logic

The previous logic failed when there are not artifacts for archiving,
for multiple reasons. Fix all of them:

* "allow-empty" needs to be set, so jenkins doesn't fail the job when it
  can't find any artifacts to archive.
* The "artifacts" string must not be empty even if "allow-empty" is set,
  as otherwise it assumes a configuration error.
* The jenkins jobs fails in the archiving step if the workspace dir
  doesn't exist.

Fixes: 4c392b9e ("jobs/gerrit: archive osmo-ccid-firmware artifacts")
Change-Id: If7a051d7be2beb617c2eda4fd1a669565c2c5b55